High Resolution Cavity Ringdown Spectroscopy And Its Application

The molecular spectroscopy in gas phase has been a significant window to under-stand the molecular internal structure and properties, and has played an important role in physical and chemical research. The transition frequency, intensity and line pro-file can be directly retrieved from the gas-phase direct absorption spectroscopy, which is so far commonly applied in analytical chemistry and physical chemistry. Among the direct absorption techniques, the cavity ring-down (CRD) technique is inherited with high sensitivity and simple implementation, and becomes more and more popular. The present work is mainly devoted to the construction of the cavity ring-down spec-trometer and its application in the absorption spectroscopy studies of some polyatomic molecules.The first chapter of the thesis presents the principle of the cavity ring-down spec-troscopy. The history and development of CRDS was briefly reviewed. The second chapter is devoted to different components of the cavity ring-down spectrometer:laser source, mode matching, design of resonance cavity, signal detecting, data acquisition, and fitting of exponential decay curve. The probl ms encountered in the construction were also discussed. In order to evaluate its performance, the line profile of the acety-lene R(9) transition centered at 12695 cm-1 was recorded. Finally, the sensitivity of the present cavity ring-down spectrometer was determined as 7×10-11/cm, and the resolution was as high as 0.0001 cm-1. The HWHM (Half width at Half Maximum) of the instrumental profile was estimated as 4x 10-5 cm-1, which reaches the limit of the laser line width. In the third chapter, the spectrometer was applied to record the 00006-0000 and O116-0110 bands of 15N14N16O and 14N15N16O in the near-IR region, the rotational assignments and band intensities were retrieved. Three (?)bands of 13C2H2 between 12240 and 12350 cm-1 were also revisited, the results showed bet-ter precision and better sensitivity than that of previous cavity ring-down spectroscopy and intra-cavity laser absorption spectroscopy studies. The Fourier transform spec-trum analysis between 3500 and 9000 cm-1 of 15N15N16O was also presented in the third chapter.6523 transitions were observed and assigned based on the global effec-tive Hamiltonian model. The band-by-band analysis led to the determination of the ro-vibrational parameters of a total of 65 bands. Among these bands,46 were newly observed, and the rotational analysis of 19 others were significantly extended and im-proved.In the chapter four, we established a cryogenic cell (down to(?)N2 temperature) for direct absorption spectroscopy experiments. High resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature between 1.30 and 1.70μm (5850-7700 cm-1). The typical noise equivalent absorption was 2x10-6 cm-1 which corresponded to a detection limit of 10-25 cm·molecule-1 (at 300 K and with a 10 Torr pressure). The reduction of the rotational congestion and the line width narrowing at low temperature will help future ro-vibrational modeling of the spectrum. The measured Doppler line broadening was found to be a reliable method to determine the sample gas temperature. The accuracy of the obtained average temperature value can be improved by the statistics over a large number of analyzed transitions. The temperature evolution of methane absorption in a selected spectral region during the cooling and warming of the cell was continuously measured, and validated the method used to retrieve data from the spectra. In total,1350 and 3473 transitions were obtained, and the lower energy levels of 845 and 1273 transitions were determined in the 5852-6181 cm-1 and 7351-7655 cm-1, respectively. The methane absorption spectrum at temperature between 80 K and 296 K can be simulated from these data. The results will be helpful to model the methane spectrum in the considered region which are of primordial importance in planetology.